<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>12扩展内置对象</title>
  </head>
  <body>
    <script>
      console.log(Array.prototype)
      Array.prototype.sum = function () {
        let sum = 0
        for (let i = 0; i < this.length; i++) {
          sum += this[i]
        }
        return sum
      }
      let arr = [1, 2, 3]
      console.log(arr.sum()) // 成功
    </script>
  </body>
</html>
